Response Of Vegetation Trends To Nature And Human Factors And Its Prediction

Vegetation is an important component of terrestrial ecosystems,accounting for76%of the land surface area.It is necessary to reveal the vegetation changes and their response to nature and human factors.Vegetation change is non-linear and non-stationary,which is affected by natural and human activities alone and interactively.Previous studies have revealed the linear trend of vegetation change,but ignored the potential trend evolution of vegetation.Previous studies have revealed the relationship between driving factors and vegetation,and quantitatively distinguished the relative importance of climate change and human activities on vegetation change.However,it is ignored to distinguish the nonlinear effects and relative importance of different natural and human factors on vegetation change.Meanwhile,the prediction of vegetation trend change under different climate and land use change is insufficient.Using the two-band enhanced vegetation index(EVI2)as the proxy for vegetation growth,we investigate the nonlinear trends of vegetation and its nonlinear relationships to nature and human activities for the area along the Silk Road Economic Belt(SREB)using the ensemble empirical mode decomposition(EEMD)and Boosted Regression Tree(BRT)methods.Furthermore,the sustainability of the vegetation trends under the Shared Socioeconomic Pathways(SSP245 and SSP585)at different time periods(2030s,2050s)is predicted.Moreover,based on the pure,shared and coupling effects by Venn's diagram(PSCV)method,we separate the pure,shared,and coupling effects of climate change and land use changes on the sustainable changes of vegetation trends.Results indicated that:(1)23.40%of the study area experiences a greening trend,mainly locates in the south of Europe and Northwest China.In addition,26.32%of the vegetation trend has been reversed,and the main shift is greening to browning reversal(19.90%),mainly distributes in countries such as Ukraine,Belarus and Kazakhstan in Europe and Central Asia.Among different vegetation types,the proportion of greening trend in forests is the highest(37.69%),mainly in southern Europe and Northwest China.The percentages of greening to browning reversal in farmland and forest are relatively high(23.10%and 24.51%,respectively),mainly locates in the east and north of Europe.The areas experience significant turning points mainly distributes in eastern Europe and parts of Central Asia,and 70.45%of them occurs after 2000.(2)The nonlinear trend of vegetation varies with different latitude,longitude,elevation,and vegetation types.At different latitudes and longitudes,the rate of greening trend in vegetation increases faster in south of 45°N and around 85°E in Northwest China,and in the near 40°E in Europe,and increases rapidly after 2005.The growth rate of greening to browning reversal decreases after 2005,especially in the north of 45°N and between 10-110°E,indicating that vegetation in the northern and central parts of the study area is more likely to shift from greening to browning.For different altitudes,the growth rate of greening trend in vegetation increases faster at altitudes between 1000–3000 m,mainly locates in Northwest China and southern Europe,while the growth rate above 3000 m is relatively slow,and the growth rate below 1000 m remains relatively stable.In addition,the greening to browning reversal mainly occurs in 2005,and mainly distributes in the area of 0-500 m,indicating that the greening to browning reversal mainly occurs in low-altitude areas,especially in plains and low hills in Europe and eastern of Central Asia.For different vegetation types,the growth rate of the greening trend in sparse vegetation and grassland increases faster.The growth rate of greening to browning reversal in the forest and farmland decreases rapidly after 2005.(3)The nonlinear change of vegetation trend is affected by the independent or interactive effects of climate and human activities.For vegetation with greening trend,land use changes(LUCC,21.87%),carbon dioxide emissions(CO₂,7.17%),and the precipitation of warmest quarter(BIO18,6.99%)are relatively more important.The increased cropland and cropland and appropriate increased CO₂ are the main factors for the greening trend of vegetation in Northwest China.The increase in precipitation in the warmest season and the increased forest are the main factors for the greening trend of vegetation in central and southern Europe.For greening to browning reversal,elevation(ELEV,11.72%),the mean temperature of warmest quarter(BIO10,9.42%),annual mean temperature(BIO1,6.59%),and the annual precipitation(BIO12,6.59%)have the relatively high contributions.Low elevation and the rapidly increased BIO1and BIO12 are the main reasons for the greening to browning reversal in Europe,while increased BIO10 is the main factor for vegetation degradation in Central Asia.There are interactions between different driving factors.For vegetation with greening trend,the interactions of the soil moisture and precipitation of warmest quarter(SOIL and BIO18,13.96%),carbon dioxide and average mean temperature(CO₂ and BIO1,9.36%),and soil moisture and daily snow water equivalent(SOIL and SWE,9.21%)are relative higher.The interaction between SOIL and BIO18 with an increased trend,the interaction with a slowly increased trend of CO₂ and an increased trend for BIO1,and the interaction between SOIL and SWE with the increased trends are the main factors for monotonic greening trends of vegetation in central and southern Europe.The interaction of the increased trends of SOIL and SWE is the main reason for the greening trends in Northwest China.In vegetation shifted from greening to browning,the strongest interactions are the driest season precipitation and the warmest month's extreme high temperature(BIO17 and BIO5,17.84%),the vapor pressure deficit and the warmest month's extreme high temperature(VPD and BIO5,12.12%),and soil moisture and daily Snow water equivalent(SOIL and SWE,7.8%).The interaction of increased rates of BIO17 and BIO5 and the interaction of increased rates of VPD and BIO5 are the main factors that cause the vegetation shifted from greening to browning in Europe.The interaction of increased rate of SOIL and slowly increased rate of SWE is the main reason for the vegetation shifted from greening to browning in Central Asia.(4)Predicting the sustainability of vegetation trends under different scenarios in the future can provide a theoretical reference for the sustainable development of the Silk Road Economic Belt.However,previous predictions did not consider both climate change and changes in land use caused by human activities.Under the current scenario,the percentage of the greening trend in vegetation is 28.13%,16.47%of the vegetation shows a sustainable greening trend,and the percentage of the increased greening trend(11.66%)is greater than the decreased(6.86%).The sustainable and increased greening trend mainly distributes in southern Europe and the eastern part of northwest China.The proportion of the greening to browning reversal is 20.53%.11.83%of the vegetation is sustainable greening to browning reversal,and the increased greening to browning reversal is 8.69%,mainly distributes in eastern Europe and some parts of Central Asia.The percentage of the decreased greening to browning reversal is 7.27%,mainly distributes in southern Europe and eastern northwest China.In the SSP245 scenario,the proportion of the greening trend in vegetation increases from 26.18%to 32.84%in 2030s and 2050s,and the sustainable greening trend in vegetation increases from 9.84%to 12.07%,distributes in the eastern part of Northwest China and the Middle East,which mainly caused by the pure effect of climate change and shared effect.The percentage of the increased greening trend is 20.77%,mainly distributes in southern Europe and northwest China.It is mainly caused by the pure effect of climate change.The percentage of greening to browning reversal under this scenario decreases from 22.78%to 16.66%in 2030s and2050s.The shared effect is the main factor leading to the decreased greening to browning reversal.The shared effect is the main reason.In the SSP585 scenario,the percentage of the greening trend decreases from 15.36%to 13.42%,of which the sustainable greening trend is nearly 6%,but the proportion of decreased greening trend is as high as 17.61%,mainly distributes in eastern Europe and northern Central Asia,and mainly caused by shared effect.The proportion of greening to browning reversal caused by the shared effect,increases to 33.43%in 2050s,of which the sustainable greening to browning reversal is about 7%,mainly distributes in eastern Europe.It mainly caused by the pure effect of climate change and the shared effect.The increased greening to browning reversal increases from 21.47%to 25.61%in2030s and 2050s,mainly distributes in southern Central Asia and eastern Europe.It is also caused by the pure effect of climate change and the shared effect.This paper reveals the nonlinear trend analysis is very important for correct assessment of vegetation change,and avoid underestimating the potential risk of vegetation degradation.Exploring the nonlinear response of vegetation trend to different natural and human activities can quantitatively evaluates the relative importance and interactions of different natural and human activity factors,and predicts the sustainability of vegetation under different scenarios.Moreover,based on the PSCV method,we separate the pure,shared,and coupling effects of climate and land use changes on the sustainable changes of vegetation trends.It will provide an early warning mechanism for the ecosystem of the area along the Silk Road Economic Belt when facing future possible challenges.Furthermore,it will provide strategic support to ensure the sustainable development of economy and ecology.